Numerous methods have been tried in the past to correlate data and determine the stability of an oceangoing vessel. This problem usually becomes of magnitude when dealing with large ships, namely those having at least one thousand gross ton displacement.
In most cases, due to the comprehensive design calculations and tests that are made during the design and production of these vessels, the moments of stability during normal operation, both in the longitudinal as well as the transverse direction, usually are not of any major concern.
Various design considerations are emphasized when designing a ship depending upon the intended use and the stability characteristics that are normally required of that ship. For example, a battleship in order to provide a stable gun platform must have a stability which prevents a continuous rolling motion and the ability to withstand substantial rotational force which is produced by the recoil upon the firing of its guns. An aircraft carrier, on the other hand because of its height above the waterline, needs to have considerable weight positioned below the waterline to counterbalance and provide the necessary platform stability. This is especially true in the longitudinal direction for the aircraft carrier because of the necessity for aircraft to take off and land as safely as possible.
Commercial cargo ships, especially those which are designed to carry liquid cargo, such as oil tankers, have special design problems due to the fact that the cargo itself provides significant ballast which effects the stability of the vessel. These factors are controlled during the loading operation where it is necessary to determine how the cargo will be loaded, in what sequence, and in what quantities. In order to accomplish and maintain the stability and trim, it may be necessary to add additional ballast to the ship or to strategically control cargo positioning to maintain the necessary stability especially if rough seas are anticipated.
In order to set the stage for the complete description of this invention, it is necessary to fully understand the terminology and laws of physics which are of concern. Although this description is not intended to be a complete explanation, it will provide the necessary background for understanding.
A floating body is acted upon by many forces, not the least of which are the forces of gravity and buoyancy. Stability is a result of these various forces which act upon the hull of the ship. When a ship is tilted or heeled by some disturbing force, it either tends to return to its original upright position or else to overturn. This tendency to rotate one way or the other is referred to as stability. The tendency to produce rotation in the ship is expressed as a moment and therefore stability is actually a moment tending either to restore the ship to its normal position or to overturn the ship.
Although gravitational forces act everywhere upon a ship, it is not necessary to attempt to consider these forces independently. Instead, we regard the total force of gravity on the ship as a single resultant or composite force representing the total weight of the ship which acts vertically downward through the ship's center of gravity (G).
Similarly, the force of buoyancy may be regarded as a single resultant or composite force which acts vertically upward through the center of buoyancy (B) located at the geometric center of the ship's underwater hull. As long as the center of gravity is above the center of buoyancy and both are aligned on a vertical plane through the longitudinal center of the ship or vessel it is said to be stable.
The real problems concerning stability occur when these forces no longer act in the same vertical plane. A vessel of this type can be disturbed from rest by many different influences, i.e. wave action, wind, turning forces created by the rudder, the addition or removal of off-center loads or cargo and the impact and damage caused by a collision or an enemy hit. These influences exert what are called heeling moments which may be temporary or possibly could be constant. A stable vessel does not capsize when subjected to these disturbances because when inclined, it develops a tendency to right itself called a righting moment (RM). A righting moment is actually equal to the righting arm (GZ) times the weight (W) or displacement of the ship. Since the displacement actually remains constant as the ship heels, the stability of the ship may be measured by the righting arm at any given heel angle.
Another factor which is involved with the question of stability is a term called metacenter (M) and the height of the metacenter (GM) above the center of gravity. When a ship is caused to heel, the center of buoyancy will shift either to starboard or port from the vertical axis.
With the ship at a given draft or depth in the water, the metacenter is the point of intersection of two successive lines of action of the force of buoyancy as the ship is heeled through various angles. The location of the metacenter depends upon how the center of buoyancy moves when the ship heels and for a small angle will usually remain on the centerline or plane of the vessel but with a large angle of heel moves either to the port or starboard side of the centerline depending upon the configuration of the hull.
The metacentric height (GM) is an indicator of the stability of the ship. In naval vessels large metacentric height (GM) and large righting arms (GZ) are desirable for resistance to damage. On the other hand, small GM dimensions are sometimes desirable for slow easy roll which makes for more accurate gunfire. As a result the GM for a naval ship is usually the result of direct compromise. With respect to stability, it is obvious that when the center of gravity is below the metacenter, the GM dimension is positive and correcting righting arms and moments develop. On the other hand, however, when the center of gravity is above the metacenter the GM is negative and upsetting or overturning moments develop. Thus, the GM dimension is an indicator of the magnitude of the stability moments and whether stability is positive or negative for the vessel.
The stability curve is a handy tool for determining the theoretical stability of a vessel. It is possible for ship designers by mathematical and graphic means to compute the righting moment of the ship at any angle of heel. The graph is formed by plotting a series of the moments which are calculated for various angles of heel. As is usual the curve indicates that as the ship heels over, it develops righting moments which gradually increase, reach a maximum and then diminish. At the same time, the stability curve applies equally to either a port or starboard roll. The initial curve holds true only for the initial stability of the ship which is determined by the original displacement and the specified distribution of the cargo, fuel, potable water, and other necessary items carried onboard a vessel. Any time a new condition exists such as when the ship sustains damage during battle or during collision or possibly runs aground, a new curve must be made to define the changed stability condition.
An important factor involved with the stability of a ship and which is a factor in the plotting of the stability curve is the draft of a ship which directly effects the righting moments. A change of draft will cause a change in the center of gravity, metacentric height and will also result in altered righting moments throughout the range of stability. This becomes critical under damage conditions and is also an important factor when loading a cargo vessel.
Another important factor when considering the stability of a vessel and the stress capability of the structure is the trim of the vessel. Trim is the difference between the drafts at the bow and stern of the vessel. Thus, when the ship trims, it inclines or tilts about an axis through the geometric center of the waterline plane which is known as the center of floatation. This trim directly effects the longitudinal stability of the ship. If a ship is out of trim by a small amount, this is not of concern, but if any large trim variations occur, this can directly effect the overall longitudinal stability of the ship. Excessive or critical trim can cause the ship to plunge or sink by diving under the surface of the water.
Trim also effects the "hog" and "sag" of the ship. These terms apply primarily to extremely elongated vessels such as super tankers and refers to the characteristic wherein the ship is bowed up in its midsection which is referred to as "hogging" or where it bows downward which is called "sagging". This tendency to hog or sag can induce extreme stresses in the hull girder structure of the vessel with an extreme condition causing actual shearing and breakup of the hull with subsequent sinking.
The damage and flooding of compartments in a vessel also presents other major concerns. If a watertight compartment has been breached allowing water to enter the compartment but not completely filling the compartment, a condition called "loose water" will exist in the compartment which can add other forces and disturbances. In addition, if the opening in the compartment is open to the sea which allows free passage of water in and out, this also adds additional forces and disturbing factors. These two factors are called the effect of "free surface" and "free communication". Both of these factors will greatly affect the righting moment and righting arm which directly effects the stability of a vessel.
As can be readily seen from the above discussion, the normal stability of an oceangoing vessel is inherently designed into the original configuration of the vessel. Even in operation with its full compliment of personnel, cargo and load, stability is inherently maintained within the design parameters and boundaries with a safe condition existing. Adversity, however, can radically change this situation to a point where the ship is no longer safe and in danger of plunging, capsizing and sinking. This is the distressed operational condition to which a substantial part of the present invention is directed.
This catastrophic change in the stability of an oceangoing vessel can be an accepted possibility in a military or naval ship. By the same token, with a commercial vessel, it is possible that catastrophic adversity such as collision, running aground or storm at sea can produce the unsafe condition. The question which arises is what can be or should be done when this unsafe condition exists.
The two primary ways of correcting this unsafe or unstable condition is to either flood counterbalancing compartments in the vessel or to dewater or pump out water which may abnormally exist in one or more of the compartments. This action is intended to produce counterbalancing forces in the vessel which will return the vessel to a normal stable condition. When this occurs, the unsafe condition is negated.
In the past it has been common practice to guess at what countermeasures are required to return the ship to a reasonable safe condition. Using this approach has in many cases resulted in catastrophic loss and sinking of the vessel. It is very easy to counterflood a wrong compartment which would tend to overbalance in the opposite direction, causing the entire vessel to roll and to capsize. By the same token, it is possible that counterflooding of a compartment either fore or aft of the center of floatation could over exaggerate an already dangerous trim condition which could cause the ship to plunge or break-up. Thus, it is possible that a "hit or miss" approach to this situation can prove to be even more dangerous than if no corrective action is taken.
In order to eliminate the guesswork that occurs in many cases there has been an attempt in the past, both on military and commercial vessels to manually calculate the stability status of the vessel under different conditions. This is naturally a very time consuming process when considering the number of watertight compartments or tanks which are present below the waterline or damage control deck of a ship. The structural size of each compartment as well as the location of the compartment with respect to the vertical, horizontal and longitudinal axis of the vessel must be accurately determined. This is a difficult task even under normal stable conditions due to the fact that the actual loading of the individual compartments during normal operations is constantly changing or varying. It becomes almost impossible under catastrophic conditions which exist at a time of damage or collision. Under these conditions, the status of various compartments is rapidly changed by flooding or the shifting of weight which if rapid enough or of a great enough magnitude can place the vessel in extreme danger in a short period of time.
In the past, the original stability condition of a vessel was obtained by the "inclining" method. This was an attempt to physically measure and calculate the actual center of gravity and hence the stability righting moments which would be developed at various angles of heel and trim. This information was acceptable for normal operation of the vessel but is of limited value in time of change due to damage or emergency.
Improvements in these primitive methods took the form of more precise measuring and calculating of the dimensions and stability moment parameters for the compartments and tanks onboard a vessel. However, these parameters were seldom corrected or updated for various day to day changes or even if the ship underwent major alterations or modifications. The result being that usually all of the available stability information was quickly out of date and unreliable. Even with this questionable background, the real problem begins when the ship sustains damage and flooding from either battle, collision or grounding. In most emergency situations, reaction time must be measured in minutes but because of the unreliable stability information and the antiquated methods used for obtaining information and calculating new parameters, it usually takes many hours to assess the situation and take the necessary corrective action. In many cases, this amount of time is not available with the needless loss of the vessel, as well as the possible loss of lives.
Attempts have been made to manually calculate a stability data card for every watertight compartment, tank or space in the vessel. These cards include current moment arm and moment force for each compartment based on various percentages of hypothetical flooding of the compartment. Thus, the projected moments and arms for each compartment based on increments of flooding, such as one-fourth, one-half, three-quarters or totally flooded are provided on the moment stability card. At the time that damage occurs, it is necessary to physically record the damage and extent of flooding for each effected compartment and transmit this information to the damage control operator.
From the previously calculated moment stability cards the necessary moments and arms for the individual damaged compartments are then obtained and the corresponding applicable information for that particular compartment is collected on a summary sheet. By reporting and summarizing this information, the total change in the stability of the vessel in its post-damaged condition can be determined. Thus, a crude indication is provided as to what possible corrective action may or may not be feasible to return the vessel to a stable condition.
In most cases, these calculations from the time that damage might occur until some corrective action for this damage can be analyzed and taken can require a number of hours. As can be easily understood in many cases where considerable damage is sustained this amount of time is not available and the ship can be sunk or the use of the vessel can be essentially lost before corrective action can take place.
In February 1982, the applicant installed and experimented with a computerized data base system onboard the aircraft carrier U.S.S. Midway. From the platform data base that was established for this vessel it was found that projected moment parameters for each compartment under various flooded conditions could be more rapidly obtained and printed as individual moment stability cards. It was agreed that this printed card could be quickly updated and later used in time of emergency to aid in manually analyzing the stability status of the vessel. The entire process could be accomplished in a relatively shorter time of an hour or less rather than the many hours which had been required in the past.